Industrial Extractors

An alternative method of preparing source material for extraction is so-called electropermeabilization. High-intensity electric field pulses cause cell damage and release of liquid contained in the cells (10). Because of its high viscosity, oil cannot be obtained by this technique but it may be applied to fruit juice extraction.

 

 

E.  Solvent Recovery

 

The extracted cake and the product need to be liberated from any solvent residu- als by stripping or distillation procedures. Therefore, the miscella leaving the

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 14  Schematic of an expander.

 

 

 

 

 

 

 

 

 

 

 

 

 

extractor is passed through a distillation step for stripping the solvent and ob- taining a solvent-free product. This product is ready for further treatment, such as liquid extraction of undesired components, enrichment of desired ones, or chemical modification.

The exhausted meal is discharged from the extractor and transported to the desolventizing step by conveyer belts or screws. Here the solvent is evapo- rated in order to obtain a solvent-free solid residual that can be used, e.g., as animal food. Usually, the desolventized meal is toasted to increase its nutritional value as food material. Desolventizing and toasting is usually carried out in two separate steps. Lurgi offers a toaster including so-called pre-desolventizer stages at the top end of the same tower. Steam is passed from below through vapor duct trays holding the meal at various heights of the tower. Double-arm agitators move the meal atop these trays until it drops through outlet holes to the tray below. Within the trays there are also steam-heated sections for indirect heating. Using solvents that are partly miscible with water (e.g., alcohol, acetone), steam cannot be applied in view of posterior separation and solvent recovery. In this case, the so-called flash desolventizing process can be applied contacting the meal with superheated solvent.

If volatile flammable organic solvents are used, operation of extraction, distillation, and desolventizing is carried out at slight vacuum to prevent the risk of explosion in case of leakage. If less volatile solvents are used, e.g., water, high temperatures must be applied, e.g., in spray drying of the extract-solvent mixture. Often, high temperatures are disadvantageous with respect to valuable compounds such as flavors and fragrances. Since the 1930s freeze drying is increasingly applied in order to evaporate water, e.g., from the extracted solu- bles of coffee. Here temperatures of 60–85°C are applied for sublimation of water at 50 Pa from coffee that is frozen at temperatures below −30°C. The highly concentrated frozen coffee is either filled into trays that are moved onto heated plates through a vacuum tunnel or directly scrapped over heated plates by agitators. In the former case, heat transfer is worse but the latter method leads to abrasion of the solid particles.

In general, environmental protection and consumer demands afford devel- opment of solvent recovery systems that provide adequate cleaning of exhaust air and careful treatment of conserving valuable product components.

 

 

F.  Final Pressing Systems

 

For production of small amounts of edible oil, mere mechanical pressing may be suitable. Physical limits of the minimal residual oil content exist due to strong adhesive forces. In case of rapeseed, a content of 7% of oil remaining in the solid matrix can be achieved only by pressing. Further treatment using solvents is not economic. Accounting for the loss in residual oil compared with a content

 

 

 

 

 

 

 

 

of 1.5% after combined prepressing and solvent extraction and high costs of maintenance in the case of full pressing, production costs of oil coming from either process are usually equal at production rates above 500 t/a.

Fruit juices are commonly produced only by mechanical treatment. Belt filter presses, e.g., manufactured by Flottweg or Zentrifuges from Westphalia, both in Germany, are used to mechanically extract juice, such as from apples and cherries. In processing of citrus juice, crushing of the fruit must be avoided since oil and bitter compounds originating from the peel have to be kept low in the juice. The FoodTech Extractor System from FMC, Florida, extracts juice and oil simultaneously by scratching off the peel using two approaching cups while the peeled fruit moves into a strainer tube. Here the juice is separated from the seeds and the rest of the fruit. In general, around half of the weight remains as solid residue subsequently processed to cattle feed supplement. Fur- thermore, about 0.4% is formed by citrus oil situated in the peel. Next to press- ing of the peel, water is used for rinsing the oil, which on its turn requires separation of the obtained emulsion by centrifugation. Quality of peel oil de- pends on the amount of water used. While the oil obtained directly from press- ing may be sold as cold-pressed citrus oil, the oil obtained from a subsequent aroma recovery step using multistage evaporation and distillation, and  thus lower in quality is applied for production of soaps and detergents. Within the juice extractor offered by Brown International Corp., the peel oil is obtained before juice extraction. The peel is cut and rinsed with water. Centrifuges carry out separation of the resulting emulsion.

 

 

G.  Complete Extraction Plants

 

Figure 15 shows a flow diagram of a hop extraction plant using ethanol as solvent. Cone hops is fed to a double-deck Rotocel extractor with 16 compart- ments after being dried. The miscella drawn from the extractor is concentrated by passing a four-stage vacuum evaporator at gentle temperatures. Complete elimination of the alcohol is carried out in a posterior separation step. The spent hops discharged from the extractor is desolventized in a dryer and subsequently pelletized for animal feed application. Recovered ethanol-water mixture is ad- justed to the desired composition by rectification and recycled. Compared to alternative CO₂ extraction (discussed in the next section), ethanol possesses little selectivity resulting in a product that contains almost the natural composition of extractables given by the feed material.

 

 

II.  SUPERCRITICAL FLUID EXTRACTION (SFE)

 

Extraction from solid material using supercritical fluid extraction (SFE), espe- cially carbon dioxide (CO₂) extraction, is established on an industrial scale for

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 15  Flow diagram of a hops extraction plant using ethanol as solvent. (Courtesy of Hallertaler Hopfenveredelungs- gesellschaft.)

 

 

 

 

 

 

 

 

a wide range of applications. Besides some large plants used for industrial pro- duction of decaffeinated coffee and hops extract, there are a number of smaller multipurpose plants that obtain extracts from a variety of natural materials, such as spices, herbs, and valuable vegetable/essential oils (11).

 

 

A.  Extraction System

 

In Fig. 16 a general flow sheet of a supercritical fluid extraction is shown. Such a processing line mainly consists of a pressurizing device, a pressure vessel for extraction, one for separation (i.e., solvent recovery), and a couple of heat ex- changers.

If separation is performed in the most common way by pressure release, the fluid must be vented through a butterfly valve before entering the separator. Separation is carried out at lower pressure than extraction. The type of pressuriz- ing device used to recirculate the fluid by rising pressure back to extraction conditions depends on the state of the solvent fluid coming from the separator. If the pressure in the separator is high enough for the fluid to be liquified by chilling in a reasonable temperature range, piston pumps may be applied. On the other hand, gas compressors working with much lower volumetric efficien- cies are needed if the fluid at the pump inlet is in a gaseous state. Nevertheless, the corresponding low separation pressures might be of interest for highly vola- tile components that are well solubilized by the compressed fluid. Pressure must be reduced considerably for precipitating these solutes. Alternative methods of solvent recovery also exist for a complete isobaric solvent cycle. The solute can

 

 

 

 

Figure 16  General flow sheet of supercritical fluid extraction.

 

 

 

 

 

 

either be absorbed by an additional liquid, e.g., water, or adsorbed on a fixed bed, e.g., of activated carbon. Therefore, the pump is just needed for maintaining fluid flow and overcoming relatively low-pressure drops along the processing line. For arriving at the operating pressure and compensating solvent losses, a relatively small additional pump has to be installed. The solvent cycle is com- monly represented by a T-S diagram from which energy balances may be drawn that are needed for heat exchanger design. For a usual SC-carbon dioxide (CO₂) extraction with liquid CO₂   at the pump inlet, Fig. 17 shows this clockwise turn- ing cycle in such a T-S diagram.

After being liquified in the chiller C1 the fluid is compressed by the pump P1. Pressure increase is supposed to take place nearly reversibly (isentropically). HE1 heats up the fluid to extraction temperature. Adiabatic expansion (no sig- nificant change in enthalpy) occurs in the butterfly valve RV1. Due to the Joule- Thompson effect, the carbon dioxide is cooled down arriving at saturation con- ditions. For better separation, the loaded fluid is completely evaporated just before entering the separator in HE2.

Besides some large-scale plants for extraction of α-acids from hops (12), there are quite a number of smaller, multipurpose plants for obtaining extracts of a variety of natural materials such as spices, herbs, and solids containing flavors or fragrances, all of them using similar setups as depicted in Fig. 16 (13). For

 

decaffeination of green coffee beans, either the beans are moistened or moist carbon dioxide is used (14). Separation and regeneration of the supercritical solvent after decaffeination is usually performed by isobaric adsorption on acti- vated carbon or absorption by water (15). In a similar way, caffeine-free tea can be produced (16). An alternative procedure has been proposed by Buse GmbH for decaffeination of green coffee beans using water saturated with carbon diox- ide at pressures of up to 30 MPa (17).

 

 

B.  Batch and Continuous Processing

 

Up to now, supercritical extraction of solids at industrial scale is mostly per- formed in a discontinuous manner due to the lack of reliable sluice systems for continuous inflow and outflow of solids to and from high-pressure vessels. In order to save expensive operation time, two to four extractors are often operated in turns. Figure 18 schematically shows a plant for solid extraction containing two extractors and an optional second separator in case stepwise pressure reduc- tion is necessary. Figure 19 shows a time schedule of discontinuous supercritical extraction including charging and discharging of four vessels in turns. One ves- sel is always off-line for depressurizing, discharging, charging, purging, and pres- surizing.

Different methods have recently been proposed to achieve continuous charging and discharging of solids to and from high-pressure vessels. By using screw conveyers or extruders, the solid is compacted entering into the extractor (18, 19). The resulting pressure drop along the moving solid bed prevents pres- sure loss while charging the extractor. An alternative multistage sluice system containing various compartments on an axially moving frame has problems of scaling up due to lack of appropriate sealing systems. In case a slurry is formed, continuous processing can be carried out using common piston and membrane pumps (20). The solid–liquid dispersion is introduced into rather tall and thin extractors or columns where the supercritical fluid is directed either co- or coun- tercurrently with respect to the liquid flow. Anyway, using a liquid “carrier” enhances mass transfer resistance from the solid particles into the supercritical fluid phase, which can partly be overcome by finely dispersing the liquid within the supercritical solvent.

Finally, discharging of solid powders formed inside the extractor by dis- solving the original carrier phase completely within the supercritical fluid may be realized by simply opening a discharge valve at the bottom and blowing out the solid particles together with the supercritical solvent. For pressure release of vessels containing, for example, pressurized carbon dioxide, one must also take into account temperature drops toward the vapor–liquid line or even surpassing the triple point that is situated at a slightly elevated pressure. As a consequence, outflow from the vessel partly takes place as a two-phase flow and blocking b 

 

 

 

 

Figure 18  Flow diagram of an extraction plant with two extractors and fractionated separation.

 

 

 

 

 

 

Figure 19  Time schedule of supercritical–solid batch extraction using four extraction vessels in turns, 120 kg per batch, 1200 kg CO₂/h at 250 atm.

 

 

 

dry ice (solidified carbon dioxide) may occur (21). The interior of the vessel is easily cooled down to −30°C (Fig. 20).

 

 

C.  Recovery Systems

 

As mentioned above, different principles of solvent (supercritical fluid) recovery have been proposed that are also applied at industrial scale. Next to the energy- consuming method of depressurizing the supercritical solvent, adsorption and absorption methods are used especially in case high amounts of supercritical

 

 

 

 

 

 

 

 

 

 

 

 

Figure 20  Pressure and temperature decrease during depressurizing of moist carbon dioxide from a 50-L vessel.

 

 

 

fluid must be circulated and the substances dissolved by this solvent are destined for elimination. If the objective is to obtain a valuable product as the extract phase, separation by pressure reduction is usually most suitable. Nevertheless, caffeine that is dissolved by supercritical carbon dioxide for decaffeination of green coffee beans may be absorbed by water in some type of high-pressure liquid–fluid extraction plant such as a cocurrent spray tower, countercurrent packed column, or mixer-settler (22), concentrating the caffeine in a posterior membrane module and obtaining a product of elevated purity (23). The high- pressure countercurrent principle is described by Jaeger focusing on the wetting behavior of packing materials at elevated pressures (24).

At industrial scale, adsorption of caffeine on activated carbon dominates, but up to now the caffeine is burned for thermal recovery of the adsorbent. Each recovery implies a 10% loss of  activated carbon. Recent developments are aimed at obtaining the adsorbate without being destroyed during regeneration. The use of alternative adsorbents, such as ion exchangers (25), is also proposed to enhance processing rates or others with hydrophilic properties in order to facilitate their regeneration by water (26). For dimensioning an adsorber, one has to consider a sufficient length in order to establish plug flow within the adsorbent bed. Therefore, the minimal length-to-diameter ratio is about 10. The required length further depends on the amount of substance to be adsorbed.

 

 

 

 

 

 

 

 

 

 

 

Adsorption capacities range between 25 and 80 g/kg adsorbent. Back mixing should be prevented by keeping fluid velocities at values below a few centime- ters per second by choosing an adequate inner diameter.

 

 

D.  Pretreatment

 

Similar to conventional liquid extraction, the solid feed material has to be condi- tioned properly. Mass transfer should be favored by maintaining small particles, eventually removing skins by pealing or dehulling. On the other hand, the struc- ture of the solid bed should be porous and as homogeneous as possible. Large particles result in high void fraction, but at the same time the mass transfer from inside the particles into the bulk fluid is slowed down. Solid particles that tend to stick to each other (e.g., at high moisture content) often form channels by the fluid or even complete blocking. As a consequence, the solid material is ex- tracted heterogeneously and to a low extent. Figure 21 shows the influence of pretreatment on the kinetics of supercritical extraction of sunflower oil. Extrac- tion of dehulled and flaked sunflower seeds proceeds faster. In general, flaking

 

 

 

 

Figure 21  Kinetics of oil extraction from sunflower seeds at 50 MPa, 60°C. DHF =

dehulled, flaked; UDHM = undehulled, milled; UDHP = undehulled, pelletized.

 

 

 

 

 

 

 

 

 

 

 

appears to be an appropriate approach to pretreatment since the thin layers mini- mize transport resistance while leaving a firm and porous structure to the bed.

 

 

E.  Vessel Design

 

In supercritical extraction, pressure vessels are needed for supply and recovery of the solvent, the extraction (loading of the solvent) itself, and a number of heat exchangers. According to their dimensions and way of operation, different types of construction and factoring of these vessels are applied (27). Figure 22 summarizes the most important methods of pressure vessel construction. In gen- eral, vessels with solid walls and those containing compound (layered) walls are distinguished. Solid-walled vessels are normally produced as single forgings. Using this method, the diameter is limited to about 1 m and the wall thickness to about 150 mm. Greater heights may be obtained by joining several cylinders by circumferential welding. Wall thickness in excess of 200 mm is achieved by joining two half shells by longitudinal welding.

The technological limitations of weight and size imposed on the men- tioned methods for solid-walled vessels are overcome to a large extent by the use of vessels with laminated walls. The presence of such multiple layers while normally beneficial, can produce complications; the insertion of an adapter or a nozzle in a layered wall requires very careful design.

Working with corrosive substances, the multilayer principle has the advan- tage of separating the corrosion problem from the required pressure resistance. The inner layer is made of a corrosion-resistant material with less tensile strength, whereas the outer layers resist the high pressure. A disadvantage of this type of vessel is reduced thermal conductivity. Usually, contact between the layers is not ideal leaving small gaps that give rise to enhanced thermal resistance. How- ever, at high pressure, this effect is partially reduced due to additional pressing of the layers toward each other. Calculation of wall thickness is carried out according to national codes, such as the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME).

Especially for design of extractor vessels, rules concerning some construc- tion principles are based on experience gained in the past two decades of indus- trial application of SFE. The ratio of inner length to inner diameter influences the performance of extraction and should therefore be carefully chosen. At too low an inner diameter, wall effects become noticeable. Although axial disper- sion may be relatively low, back mixing becomes relevant within a tall extractor. On the other hand, high inner diameters may result in heterogeneous extraction with respect to the radial position. In most cases, solid extraction is performed upflow. The fluid enters the bottom of the extractor at its center. The extractor contains a so-called product basket that has a sieve tray bottom or, even better,

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 22  Methods of pressure vessel construction.

 

 

 

 

 

 

 

 

 

 

 

 

a sinter metal bottom. At its circumference, the basket must be sealed towards the inner wall of the extractor to prevent bypassing of the fluid. Since the basket enters the extractor by opening the extractor top, this top must be removable. Therefore, any fixed tubing at the top would have to be replaced for opening the extractor. Therefore, the outlet is positioned just below the top to one side of the extractor. This has to be considered for a homogeneous flow within the extractor because having the outlet on one side results in asymmetrical flow.

In the case of discontinuous charging and discharging of solids, the vessel must contain quick-acting closures. Furthermore, cleaning must be facilitated if there is a risk of accumulating precipitates.

 

 

F.  Heat Exchangers

 

The specific heat transport properties of the respective supercritical fluid and their variation depending on operating conditions must be taken into account for heat exchanger design. The rate of heat to be transferred within the fluid cycle, e.g., CO₂, may be deduced from the T-S diagram taking the respective points of specific enthalpy. Coming from the pump (Fig. 16) the supercritical fluid, e.g., CO₂, enters the heater HE1 at, say, 15°C and 30 MPa. The corre- sponding enthalpy of CO₂ amounts to about −284 kJ/kg. Raising the temperature to 100°C, which gives an enthalpy of −116 kJ/kg, 168 kJ/kg of heat needs to be transferred. At a CO₂ mass flow of 500 kg/h the transferred heat flux comes out to be 23 kW.

For cleaning purposes, heat exchangers should be constructed as tube bun- dles or double tubes. In particular, the evaporator following the throttling valve is at risk of being blocked by precipitated extract. Vertical orientation of this heat exchanger helps downflow of precipitating liquid. For the condenser posi- tion, different possibilities are discussed leaving some doubts for the best solu- tion. In the course of cooling, a condensate film is formed on the walls increas- ing heat transfer resistance to the gaseous phase. A diagonal orientation down toward the fluid outlet allows the freshly formed liquid to drop off the tube walls right away, leaving only a thin film of condensate. For laboratory-scale plants, heat exchangers may also be constructed as a coil of high-pressure tube placed in a thermostat bath to keep dimensions small despite relatively high heat transfer area.

 

 

G.  Pumps and Compressors

 

In general, pumps are used to transport liquids through pipes. Every pump has certain operating characteristics due to which mass flow and pressure increase are related for the installed piping. Figure 23 qualitatively shows operating char- acteristics for centrifugal and piston pumps. Centrifugal pumps that are usually